I don't have data, but intuition tells me that because there are less trains running on the lines, there are less holdups/delays at stations that have a domino effect across the line, and even if there are, there's more space between them for the differences between each set to be made up almost autonomously and not through 'holds' at stations or at certain signals.
Still doesn't make much sense to me to put the service cuts in place when more people are starting to return back to work, the weather's getting nicer/leisure riders are returning, and a sliver of normalcy is somewhat in sight. With the latest stimulus package, the MBTA should be crystal clear where the money they're getting is going.